The identities of the gunmen were unclear but victims' relatives blamed political rivals.
Philippine elections are particularly violent in the south because of the presence of armed groups, including Muslim rebels fighting for self-rule in the predominantly Roman Catholic nation, and political warlords who maintain private armies.
The decades-long Muslim insurgency has killed about 120,000 people since the 1970s.
But a presidential adviser, Jesus Dureza, said Monday's massacre was "unequaled in recent history."
"There must be a total stop to this senseless violence," he said, recommending that a state of emergency be imposed in the area to disarm all gunmen. "Anything else will not work."
National elections are scheduled for May 2010.
About 100 gunmen had stopped the convoy, military spokesman Lt. Col. Romeo Brawner said.
The group comprised the wife of Buluan township Vice Mayor Ismael Mangudadatu, along with his two sisters, followers and several local journalists. They were traveling to nearby Shariff Aguak township to file Mangudadatu's nomination papers for the position of governor of Maguindanao province, Brawner said.
Mangudadatu, who was not in the convoy, said his wife and relatives were among the dead. He accused his political rivals belonging to a prominent clan for the massacre. Representatives of that family did not comment on the allegations.